This invention relates to a relief valve apparatus for an oil pressure operated clutch and is, more particularly, directed to a relief valve apparatus which insures reliable clutch operation.
Generally, an oil pressure operated clutch is so designed that engagement of the clutch may be accomplished by supplying oil under pressure to an oil pressure operation chamber. In such a clutch, the oil in the oil pressure operation chamber is subjected to centrifugal force at the time the clutch is being rotated and, consequently, there is a tendency that oil will be retained in the oil pressure operation chamber even if it is intended that all of the operation oil will be discharged therefrom. Under the influence of a centrifugal force, the residual oil acts on a piston of the clutch and, as a result, there is caused a dragging or partial engagement of the clutch. This tendency is especially pronounced where the oil pressure operation chamber is large in diameter and the clutch is high in rotation speed.
As one of the means for discharging this residual oil, there has been heretofore known a leaf spring type relief valve. This leaf spring type valve, however, requires that the valve opening force of the spring be sufficiently large that the valve can open even at the highest speed of rotation of the clutch, in order to overcome the centrifugal force of the residual oil in the oil pressure operation chamber, even when the supply pressure becomes zero. However, when oil under pressure is being supplied to the oil pressure operation chamber, the relief valve remains open until the oil pressure in the oil pressure operation chamber exceeds that required to close the valve. As a result, the drag time of the clutch is prolonged and wear of clutch friction plates is accelerated. Additionally, the oil continues to leak until the check valve closes, this resulting in a pressure drop in the oil pressure system.
If, in an attempt to overcome the above disadvantages, the oil pressure pump is made larger in capacity, this will cause other problems, e.g., the pump becomes a load after engagement of the clutch and a larger pump will cause a lowering in efficiency of the system. Furthermore, a large amount of the operation oil must be rapidly supplied into the oil pressure operation chamber for closing the relief valve at a stroke; consequently, the oil pressure is rapidly increased in the oil pressure operation chamber and results in a large engaging shock. These difficulties are emphasized at the time of low speed rotation of the clutch at which time the action of the centrifugal force is small.
Accordingly, the present invention may be viewed as a response to the above problems in the prior art.